The present invention relates to a control apparatus and control method for reducing agent supply apparatuses supplying a liquid reducing agent derived from ammonia to the exhaust passage of an internal combustion engine.
There is a known exhaust purifying apparatus including a selective reduction catalyst disposed in the exhaust passage of an internal combustion engine and a reducing agent supply apparatus for injecting a liquid reducing agent derived from ammonia such as a aqueous urea solution disposed upstream of the selective reduction catalyst as one of an exhaust purifying apparatuses purifying NOX in an exhaust gas exhausted from an internal combustion engine such as a diesel engine installed in vehicle or the like. This exhaust purifying apparatus performs an efficient reduction reaction between NOX in an exhaust gas and ammonia in the selective reduction catalyst and decomposes the NOX into in nitrogen, water, and so on.
When using as an aqueous urea solution as a liquid reducing agent, the aqueous urea solution is adjusted to have a concentration giving the lowest freeze temperature to minimize the freezing of the aqueous urea solution. However, since the freeze temperature of the aqueous urea solution is at least −11 degrees or so, the aqueous urea solution may be frozen while the reducing agent supply apparatus stops supplying the aqueous urea solution in cold climate areas or the like. When the liquid reducing agent is frozen, the unfreezing control of the frozen liquid reducing agent is performed to enable injection control (see JP-A-2008-180110, for example).